Strain sensors

ABSTRACT

A strain sensor can include a resistor, a first electrical contact at a first end of the resistor, and a second electrical contact at a second end of the resistor. The resistor can be formed of a matrix of sintered elemental transition metal particles interlocked with a matrix of fused thermoplastic polymer particles.

BACKGROUND

Strain sensors are commonly used to measure strain, or in other words,the amount of compression or extension of an object under an appliedforce. Many types of measurement devices use a strain sensor to measureanother quantity, such as pressure, load, torque, and weight. A commontype of strain sensor utilizes the common property that the electricalresistance of a material depends on the material's length andcross-sectional area. Strain sensors often consist of a metal foilpattern bonded to a substrate material. Such strain sensors are placedin such a way that the strain being measured will cause the metal foilpattern to flex. Depending on the direction of flexing, the metal foilpattern can be subjected to compressive or tensile forces. When themetal foil pattern is compressed, the length of the metal foil patternis slightly reduced and the thickness is slightly increased, whichresults in an overall reduction in resistance. When the metal foilpattern is under tensile stress, the length of the metal foil patternincreases slightly and the thickness is slightly reduced, resulting inan increase in resistance. These small changes in resistance can bedetected and correlated to the strain experienced by the strain sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a 3-dimensional printed strain sensor inaccordance with examples of the present disclosure;

FIG. 2 is a top plan view of a 3-dimensional printed part having anintegrated strain sensor in accordance with examples of the presentdisclosure;

FIG. 3 is a close-up side cross-sectional view of a layer ofthermoplastic polymer powder with a conductive fusing ink printed on aportion of the layer in accordance with examples of the presentdisclosure;

FIG. 4 is a close-up side cross-sectional view of the layer of FIG. 3after the layer has been cured in accordance with examples of thepresent disclosure;

FIG. 5 is a flow chart illustrating a method of making a 3-dimensionalprinted part having an integrated strain sensor in accordance withexamples of the present disclosure;

FIG. 6A is a side plan view of a 3-dimensional printed part having anintegrated strain sensor in accordance with examples of the presentdisclosure; and

FIG. 6B is a front plan view of the 3-dimensional printed part of FIG.6A rotated 90 degrees in accordance with examples of the presentdisclosure.

The figures depict several examples of the presently disclosedtechnology. However, it should be understood that the present technologyis not limited to the examples depicted.

DETAILED DESCRIPTION

The present disclosure is drawn to the area of strain sensors and 3Dprinting. More specifically, the present disclosure provides3-dimensional printed strain sensors, 3-dimensional printed parts withintegrated strain sensors, and methods of printing the parts.

Typical strain sensors are discrete components that are normallyfastened to a surface under investigation using adhesives or fasteners.For example, to measure strain in a structural beam, strain sensors canbe placed on surfaces of the beam transverse to the direction the beamis expected to bend in order to measure flexion of the beam.

The present technology provides strain sensors. These strain sensors canbe integrated during the design of a 3-dimensional printed object andprinted as a part of the finished object itself. Thus, strain sensorcomponents can be integrated to a part without using adhesives orfasteners. By eliminating the separate strain sensor component, thecomplexity and cost of a system using the strain sensor can be reduced.Potential cost efficiencies include the elimination of manufacturingsteps and material costs. The present technology can also make systemsmore durable and reliable by removing the mode of failure due toadhesive delamination or fastener failure. Integrating strain sensors asa part of 3-dimensional printed objects also allows for measuring ofinternal strains at any internal location in the part. This can provideadvantages over a separate strain sensor attached on the outside of theobject, which only measures strain at the external surface where thesensor is attached.

3-dimensional printing can be used to form a wide variety of parts,including parts that would be difficult or impossible to manufactureusing traditional methods such as milling or molding. The presenttechnology allows 3-dimensional printed strain sensors to be printedintegrally in any 3-dimensional printed part. In some cases, the strainsensor can provide information to monitor the 3-dimensional printed partfor stresses to which the part is subjected. In some examples, such apart can be manufactured to less stringent strength specifications thana non-instrumented part because the constant feedback from the strainsensor provides for detection of stresses that may compromise theintegrity of the part. Using this feedback, the system in which the partis used can be adjusted to reduce stress on the part before the partfails. Thus, the present technology can reduce the part manufacturingcost without degrading performance of the system in which the part isused. The strain sensor can also improve accuracy of the strainmeasurement. Because the strain sensor is an integral component of thepart being tested for strain, the strain sensor is always reliablyexposed to the same forces exerted on the part.

In some examples of the present technology, 3-dimensional printed partswith integrated strain sensors can be formed using a process involving apowder bed of thermoplastic polymer particles, a conductive fusing ink,and a second fusing ink. In this process, a thin layer of polymer powderis spread on a bed to form a powder bed. A printing head, such as aninkjet print head, is then used to print fusing inks over portions ofthe powder bed corresponding to a thin layer of the three dimensionalobject to be formed. A conductive fusing ink can be printed in areaswhere a conductive resistor for the strain sensor is desired to beformed, and a second fusing ink can be printed in other areas. The bedis then exposed to a light source, e.g., typically the entire bed. Thefusing inks absorb more energy from the light than the unprinted powder.The absorbed light energy is converted to thermal energy, causing theprinted portions of the powder to melt and coalesce. This forms a solidlayer. After the first layer is formed, a new thin layer of polymerpowder is spread over the powder bed and the process is repeated to formadditional layers until a complete 3-dimensional part is printed. Such3-dimensional printing processes can achieve fast throughput with goodaccuracy.

With this description in mind, FIG. 1 shows an example of a strainsensor 100. The strain sensor, which can be a 3-dimensional printedstrain sensor, can include a resistor 110, a first electrical contact120 at a first end of the resistor, and a second electrical contact 130at a second end of the resistor. The resistor can be formed of a matrixof sintered elemental transition metal particles interlocked with amatrix of fused thermoplastic polymer particles. This composite materialincluding the matrix of sintered elemental transition metal particlesand the matrix of fused thermoplastic polymer particles will bedescribed in more detail below.

The present technology also extends to 3-dimensional printed partshaving an integrated strain sensor. FIG. 2 shows an example of such a3-dimensional printed part 200. The part can include a part body 210formed of fused thermoplastic polymer particles. The integrated strainsensor can include a resistor 110, a first electrical contact 120 at afirst end of the resistor, and a second electrical contact 130 at asecond end of the resistor. As described above, the resistor can beformed of a matrix of sintered elemental transition metal particlesinterlocked with a matrix of fused thermoplastic polymer particles. Thematrix of fused thermoplastic polymer particles can be continuouslyfused to the fused thermoplastic polymer particles of the part body.

The conductive composite material making up the resistor is shown inmore detail in FIGS. 3-4. As shown in FIG. 3, the resistor can be formedfrom thermoplastic polymer particles 330 and transition metal particles320. In one example of a method for making the resistor, a layer 300 ofthermoplastic polymer particles can be spread in a powder bed3-dimensional printer. A first portion 310 of the layer can be printedwith a conductive ink containing a transition metal. A second portion340 can be printed with a fusing ink that includes a fusing agentcapable of absorbing energy from electromagnetic radiation andconverting the energy to heat. The layer of thermoplastic polymerparticles can then be exposed to electromagnetic radiation to raise thetemperature of the layer, causing the polymer particles to fuse togetherand the transition metal particles to sinter together. FIG. 4 shows thelayer 400 after fusing. The polymer particles fuse together to form amatrix of fused thermoplastic polymer particles 430, and the transitionmetal particles sinter together to form a matrix of sintered transitionmetal particles 420. The matrix of fused thermoplastic polymer particlesand the matrix of sintered transition metal particles are interlocked,forming a conductive composite. Additionally, the conductive compositeis present only in the first region 410 where the conductive ink wasprinted, and not in the second region 440 where the other fusing ink wasprinted.

It should be noted that these figures are not necessarily drawn toscale, and the relative sizes of powder particles and transition metalparticles can differ from those shown. For example, in many cases thetransition metal particles can be much smaller than the powderparticles, such as 2-3 orders of magnitude smaller.

It should also be noted that FIG. 4 shows only a 2-dimensionalcross-section of the conductive composite. Although the sintered metalparticles appear to be in isolated locations in the figure, the matrixof sintered metal particles can be a continuously connected matrix inthree dimensions. Thus, the conductive composite can have goodelectrical conductivity through the matrix of sintered transition metalparticles.

A variety of materials can be used to form the 3-dimensional printedstrain sensors and parts having integrated strain sensors. In someexamples, the materials can include a thermoplastic polymer powder, aconductive fusing ink, and a second fusing ink. The thermoplasticpolymer powder can include powder particles with an average particlesize from 20 μm to 100 μm. As used herein, “average” with respect toproperties of particles refers to a number average unless otherwisespecified. Accordingly, “average particle size” refers to a numberaverage particle size. Additionally, “particle size” refers to thediameter of spherical particles, or to the longest dimension ofnon-spherical particles.

In certain examples, the polymer particles can have a variety of shapes,such as substantially spherical particles or irregularly-shapedparticles. In some examples, the polymer powder can be capable of beingformed into 3D printed parts with a resolution of 20 to 100 microns. Asused herein, “resolution” refers to the size of the smallest featurethat can be formed on a 3D printed part. The polymer powder can formlayers from about 20 to about 100 microns thick, allowing the fusedlayers of the printed part to have roughly the same thickness. This canprovide a resolution in the z-axis direction of about 20 to about 100microns. The polymer powder can also have a sufficiently small particlesize and sufficiently regular particle shape to provide about 20 toabout 100 micron resolution along the x-axis and y-axis.

In some examples, the thermoplastic polymer powder can be colorless. Forexample, the polymer powder can have a white, translucent, ortransparent appearance. When used with a colorless fusing ink, suchpolymer powders can provide a printed part that is white, translucent,or transparent. In other examples, the polymer powder can be colored forproducing colored parts. In still other examples, when the polymerpowder is white, translucent, or transparent, color can be imparted tothe part by the fusing ink or another colored ink.

The thermoplastic polymer powder can have a melting or softening pointfrom about 70° C. to about 350° C. In further examples, the polymer canhave a melting or softening point from about 150° C. to about 200° C. Avariety of thermoplastic polymers with melting points or softeningpoints in these ranges can be used. For example, the polymer powder canbe selected from the group consisting of nylon 6 powder, nylon 9 powder,nylon 11 powder, nylon 12 powder, nylon 66 powder, nylon 612 powder,polyethylene powder, thermoplastic polyurethane powder, polypropylenepowder, polyester powder, polycarbonate powder, polyether ketone powder,polyacrylate powder, polystyrene powder, and mixtures thereof. In aspecific example, the polymer powder can be nylon 12, which can have amelting point from about 175° C. to about 200° C. In another specificexample, the polymer powder can be thermoplastic polyurethane.

The thermoplastic polymer particles can also in some cases be blendedwith a filler. The filler can include inorganic particles such asalumina, silica, or combinations thereof. When the thermoplastic polymerparticles fuse together, the filler particles can become embedded in thepolymer, forming a composite material. In some examples, the filler caninclude a free-flow agent, anti-caking agent, or the like. Such agentscan prevent packing of the powder particles, coat the powder particlesand smooth edges to reduce inter-particle friction, and/or absorbmoisture. In some examples, a weight ratio of thermoplastic polymerparticles to filler particles can be from 10:1 to 1:2 or from 5:1 to1:1.

A conductive ink can be used to form portions of a 3-dimensional printedpart that will act as a resistor in the strain sensor. The conductivefusing ink can include a transition metal. When the conductive fusingink is printed onto a layer of the thermoplastic polymer powder, theconductive ink can penetrate into the spaces between powder particles.The layer can then be cured by exposing the layer to electromagneticradiation. The conductive fusing ink can facilitate fusing of the powderparticles by absorbing energy from the electromagnetic radiation andconverting the energy to heat. This raises the temperature of the powderabove the melting or softening point of the thermoplastic polymer.Additionally, during printing, curing, or both, the transition metal inthe conductive ink can form a conductive transition metal matrix thatbecomes interlocked with the fused thermoplastic polymer particles.

In some examples, the transition metal in the conductive ink can be inthe form of elemental transition metal particles. The elementaltransition metal particles can include, for example, silver particles,copper particles, gold particles, platinum particles, palladiumparticles, chromium particles, nickel particles, zinc particles, orcombinations thereof. The particles can also include alloys of more thanone transition metal, such as Au—Ag, Ag—Cu, Ag—Ni, Au—Cu, Au—Ni,Au—Ag—Cu, or Au—Ag—Pd.

In certain examples, other non-transition metals can be included inaddition to the transition metal. The non-transition metals can includelead, tin, bismuth, indium, gallium, and others. In some examples,soldering alloys can be included. The soldering alloys can includealloys of lead, tin, bismuth, indium, zinc, gallium, silver, copper, invarious combinations. In certain examples, such soldering alloys can beprinted in locations that are to be used as soldering connections forprinted electrical components. The soldering alloys can be formulated tohave low melting temperatures useful for soldering, such as less than230 ° C.

In further examples, the elemental transition metal particles can benanoparticles having an average particle size from 10 nm to 200 nm. Inmore specific examples, the elemental transition metal particles canhave an average particle size from 30 nm to 70 nm.

As metal particles are reduced in size, the temperature at which theparticles are capable of being sintered can also be reduced. Therefore,using elemental transition metal nanoparticles in the conductive fusingink can allow the particles to sinter and form a conductive matrix ofsintered nanoparticles at relatively low temperatures. For example, theelemental transition metal particles in the conductive fusing ink can becapable of being sintered at or below the temperature reached duringcuring in the 3-dimensional printing process. In a further example, thethermoplastic polymer powder bed can be heated to a preheat temperatureduring the printing process, and the elemental transition metalparticles can be capable of being sintered at or below the preheattemperature. In still further examples, the elemental transition metalparticles can be capable of being sintered at a temperature from 20 ° C.to 350 ° C. As used herein, the temperature at which the elementaltransition metal particles are capable of being sintered refers to thelowest temperature at which the particles will become sintered together,forming a conductive matrix of sintered particles. It is understood thattemperatures above this lowest temperature will also cause the particlesto become sintered.

In additional examples of the conductive fusing ink, the transitionmetal can be in the form of elemental transition metal particles thatare stabilized by a dispersing agent at surfaces of the particles. Thedispersing agent can include ligands that passivate the surface of theparticles. Suitable ligands can include a moiety that binds to thetransition metal. Examples of such moieties can include sulfonic acid,phosphonic acid, carboxylic acid, dithiocarboxylic acid, phosphonate,sulfonate, thiol, carboxylate, dithiocarboxylate, amine, and others. Insome cases, the dispersing agent can contain an alkyl group having from3-20 carbon atoms, with one of the above moieties at an end of the alkylchain. In certain examples, the dispersing agent can be an alkylamine,alkylthiol, or combinations thereof. In further examples, the dispersingagent can be a polymeric dispersing agent, such as polyvinylpyrrolidone(PVP), polyvinylalcohol (PVA), polymethylvinylether, poly(acrylic acid)(PAA), nonionic surfactants, polymeric chelating agents, and others. Thedispersing agent can bind to the surfaces of the elemental transitionmetal particles through chemical and/or physical attachment. Chemicalbonding can include a covalent bond, hydrogen bond, coordination complexbond, ionic bond, or combinations thereof. Physical attachment caninclude attachment through van der Waal's forces, dipole-dipoleinteractions, or a combination thereof.

In further examples, the conductive fusing ink can include a transitionmetal in the form of a metal salt or metal oxide. Under certainconditions, a transition metal salt or metal oxide in the conductive inkcan form elemental transition metal particles in situ after beingprinted onto the thermoplastic polymer powder bed. The elementaltransition metal particles thus formed can then be sintered together toform a conductive matrix. In some examples, a reducing agent can bereacted with the metal salt or metal oxide to produce elemental metalparticles. In one example, a reducing agent can be underprinted onto thepowder bed before the conductive fusing ink. In another example, areducing agent can be overprinted over the conductive fusing ink. Ineither case, the reducing agent can be reacted with the metal salt ormetal oxide to form elemental metal particles before the thermoplasticpolymer particle layer is cured. Suitable reducing agents can include,for example, glucose, fructose, maltose, maltodextrin, trisodiumcitrate, ascorbic acid, sodium borohydride, ethylene glycol,1,5-pentanediol, 1,2-propylene glycol, and others.

The concentration of transition metal in the conductive fusing ink canvary. However, higher transition metal concentrations can tend toprovide better conductivity due to a larger amount of conductivematerial being deposited on the powder bed. In some examples, theconductive fusing ink can contain from about 5 wt % to about 50 wt % ofthe transition metal, with respect to the entire weight of theconductive fusing ink. In further examples, the conductive fusing inkcan contain from about 10 wt % to about 30 wt % of the transition metal,with respect to the entire weight of the conductive fusing ink.

In some examples of the present technology, a pretreat ink can be usedwith the conductive fusing ink. The pretreat ink can include a halogensalt, such as sodium chloride or potassium chloride, for example. Thehalogen salt can react with dispersing agents at the surfaces oftransition metal particles to remove the dispersing agents from theparticles. This can increase the sintering between the metal particlesand improve the conductivity of the matrix formed of the sinteredparticles. The pretreat ink can be dispensed onto the powder bed beforethe conductive fusing ink. When the conductive fusing ink is printedover the pretreat ink, the transition metal particles can come intocontact with the halogen salt in the pretreat ink. In alternateexamples, the polymer powder can be pretreated with a halogen saltbefore being used in the 3-dimensional printing system. When theconductive fusing ink is printed onto the powder bed, the transitionmetal particles in the conductive fusing ink can come into contact withthe halogen salt already present on the powder.

A second fusing ink can also be incorporated in the materials used tomake 3-dimensional printed strain sensors and parts having integratedstrain sensors. In some examples, the second fusing ink can be devoid orsubstantially devoid of the transition metal contained in the conductivefusing ink. Thus, the second fusing ink can provide a lower conductivitythan the conductive fusing ink when printed on the thermoplastic polymerpowder. However, in some examples the second fusing ink can includemetal particles that provide a lower conductivity than the transitionmetal in the conductive fusing ink. For example, the second fusing inkcan include metal particles with passivated surfaces that do not sintertogether to form a conductive matrix.

The second fusing ink can contain another fusing agent that is capableof absorbing electromagnetic radiation to produce heat. The fusing agentcan be colored or colorless. In various examples, the fusing agent canbe carbon black, near-infrared absorbing dyes, near-infrared absorbingpigments, tungsten bronzes, molybdenum bronzes, metal nanoparticles, orcombinations thereof. Examples of near-infrared absorbing dyes includeaminium dyes, tetraaryldiamine dyes, cyanine dyes, pthalocyanine dyes,dithiolene dyes, and others. In further examples, the fusing agent canbe a near-infrared absorbing conjugated polymer such aspoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), apolythiophene, poly(p-phenylene sulfide), a polyaniline, apoly(pyrrole), a poly(acetylene), poly(p-phenylene vinylene),polyparaphenylene, or combinations thereof. As used herein, “conjugated”refers to alternating double and single bonds between atoms in amolecule. Thus, “conjugated polymer” refers to a polymer that has abackbone with alternating double and single bonds. In many cases, thefusing agent can have a peak absorption wavelength in the range of 800nm to 1400 nm.

The amount of fusing agent in the second fusing ink can vary dependingon the type of fusing agent. In some examples, the concentration offusing agent in the second fusing ink can be from 0.1 wt % to 20 wt %.In one example, the concentration of fusing agent in the second fusingink can be from 0.1 wt % to 15 wt %. In another example, theconcentration can be from 0.1 wt % to 8 wt %. In yet another example,the concentration can be from 0.5 wt % to 2 wt %. In a particularexample, the concentration can be from 0.5 wt % to 1.2 wt %.

In some examples, the fusing ink can have a black or gray color due tothe use of carbon black as the fusing agent. However, in other examplesthe fusing ink can be colorless or nearly colorless. The concentrationof the fusing agent can be adjusted to provide a fusing ink in which thevisible color of the fusing ink is not substantially altered by thefusing agent. Although some of the above described fusing agents havelow absorbance in the visible light range, the absorbance is usuallygreater than zero. Therefore, the fusing agents can typically absorbsome visible light, but their color in the visible spectrum can minimalenough that it does not substantially impact the ink's ability to takeon another color when a colorant is added (unlike carbon black whichdominates the ink's color with gray or black tones). The fusing agentsin concentrated form can have a visible color, but the concentration ofthe fusing agents in the fusing ink can be adjusted so that the fusingagents are not present in such high amounts that they alter the visiblecolor of the fusing ink. For example, a fusing agent with a very lowabsorbance of visible light wavelengths can be included in greaterconcentrations compared to a fusing agent with a relatively higherabsorbance of visible light. These concentrations can be adjusted basedon a specific application with some experimentation.

In further examples, the concentration of the fusing agent can be highenough that the fusing agent impacts the color of the fusing ink, butlow enough that when the ink is printed on the thermoplastic polymerpowder, the fusing agent does not impact the color of the powder. Theconcentration of the fusing agent can be balanced with the amount offusing ink that is to be printed on the polymer powder so that the totalamount of fusing agent that is printed onto the polymer powder is lowenough that the visible color of the polymer powder is not impacted. Inone example, the fusing agent can have a concentration in the fusing inksuch that after the fusing ink is printed onto the polymer powder, theamount of fusing agent in the polymer powder is from 0.0003 wt % to 5 wt% with respect to the weight of the polymer powder.

The fusing agent can have a temperature boosting capacity sufficient toincrease the temperature of the polymer powder above the melting orsoftening point of the polymer powder. As used herein, “temperatureboosting capacity” refers to the ability of a fusing agent to convertnear-infrared light energy into thermal energy to increase thetemperature of the printed polymer powder over and above the temperatureof the unprinted portion of the polymer powder. Typically, the polymerpowder particles can be fused together when the temperature increases tothe melting or softening temperature of the polymer. As used herein,“melting point” refers to the temperature at which a polymer transitionsfrom a crystalline phase to a pliable, amorphous phase. Some polymers donot have a melting point, but rather have a range of temperatures overwhich the polymers soften. This range can be segregated into a lowersoftening range, a middle softening range and an upper softening range.In the lower and middle softening ranges, the particles can coalesce toform a part while the remaining polymer powder remains loose. If theupper softening range is used, the whole powder bed can become a cake.The “softening point,” as used herein, refers to the temperature atwhich the polymer particles coalesce while the remaining powder remainsseparate and loose. When the fusing ink is printed on a portion of thepolymer powder, the fusing agent can heat the printed portion to atemperature at or above the melting or softening point, while theunprinted portions of the polymer powder remain below the melting orsoftening point. This allows the formation of a solid 3D printed part,while the loose powder can be easily separated from the finished printedpart.

Although melting point and softening point are often described herein asthe temperatures for coalescing the polymer powder, in some cases thepolymer particles can coalesce together at temperatures slightly belowthe melting point or softening point. Therefore, as used herein “meltingpoint” and “softening point” can include temperatures slightly lower,such as up to about 20° C. lower, than the actual melting point orsoftening point.

In one example, the fusing agent can have a temperature boostingcapacity from about 10° C. to about 70° C. for a polymer with a meltingor softening point from about 100° C. to about 350° C. If the powder bedis at a temperature within about 10° C. to about 70° C. of the meltingor softening point, then such a fusing agent can boost the temperatureof the printed powder up to the melting or softening point, while theunprinted powder remains at a lower temperature. In some examples, thepowder bed can be preheated to a temperature from about 10° C. to about70° C. lower than the melting or softening point of the polymer. Thefusing ink can then be printed onto the powder and the powder bed can beirradiated with a near-infrared light to coalesce the printed portion ofthe powder.

In some examples of the present technology, the conductive fusing inkand the second fusing ink can be balanced so that thermoplastic polymerpowder that is printed with the conductive fusing ink and the secondfusing ink reach nearly the same temperature when exposed to lightduring curing. The type and amount of fusing agent in the second fusingink can be selected to match the temperature boosting capacity of thetransition metal in the conductive fusing ink. The type and amount oftransition metal in the conductive fusing ink can also be adjusted tomatch the temperature boosting capacity of the fusing agent in thesecond fusing ink. Additionally, in some examples the conductive fusingink can contain another fusing agent other than the transition metal. Incertain examples, the conductive fusing ink and the second fusing inkcan raise the temperature of the thermoplastic polymer powder totemperatures within 30 ° C., within 20 ° C., or within 10 ° C. of eachother during curing.

In further examples, colored inks can also be used for adding color tothe thermoplastic polymer powder. This can allow for printing offull-color 3-dimensional parts. In one example, the cyan, magenta,yellow, and black inks can be used in addition to the conductive fusingink, second fusing ink, and pretreat ink if present.

Each of the conductive fusing ink, pretreat ink, second fusing ink, andadditional colored inks can be formulated for use in an ink jet printer.The transition metal and fusing agents can be stable in an ink jet inkvehicle and the inks can provide good ink jetting performance. In someexamples, the transition metal and fusing agents can be water-soluble,water-dispersible, organic-soluble, or organic-dispersible. Thetransition metal and fusing agents can also be compatible with thethermoplastic polymer powder so that jetting the inks onto the polymerpowder provides adequate coverage and interfiltration of the transitionmetal and fusing agents into the powder.

Any of the above described inks can also include a pigment or dyecolorant that imparts a visible color to the inks. In some examples, thecolorant can be present in an amount from 0.5 wt % to 10 wt % in theinks. In one example, the colorant can be present in an amount from 1 wt% to 5 wt %. In another example, the colorant can be present in anamount from 5 wt % to 10 wt %. However, the colorant is optional and insome examples the inks can include no additional colorant. These inkscan be used to print 3D parts that retain the natural color of thepolymer powder. Additionally, the inks can include a white pigment suchas titanium dioxide that can also impart a white color to the finalprinted part. Other inorganic pigments such as alumina or zinc oxide canalso be used.

In some examples, the colorant can be a dye. The dye may be nonionic,cationic, anionic, or a mixture of nonionic, cationic, and/or anionicdyes. Specific examples of dyes that may be used include, but are notlimited to, Sulforhodamine B, Acid Blue 113, Acid Blue 29, Acid Red 4,Rose Bengal, Acid Yellow 17, Acid Yellow 29, Acid Yellow 42, AcridineYellow G, Acid Yellow 23, Acid Blue 9, Nitro Blue Tetrazolium ChlorideMonohydrate or Nitro BT, Rhodamine 6G, Rhodamine 123, Rhodamine B,Rhodamine B Isocyanate, Safranine O, Azure B, and Azure B Eosinate,which are available from Sigma-Aldrich Chemical Company (St. Louis,Mo.). Examples of anionic, water-soluble dyes include, but are notlimited to, Direct Yellow 132, Direct Blue 199, Magenta 377 (availablefrom Ilford AG, Switzerland), alone or together with Acid Red 52.Examples of water-insoluble dyes include azo, xanthene, methine,polymethine, and anthraquinone dyes. Specific examples ofwater-insoluble dyes include Orasol® Blue GN, Orasol® Pink, and Orasol®Yellow dyes available from Ciba-Geigy Corp. Black dyes may include, butare not limited to, Direct Black 154, Direct Black 168, Fast Black 2,Direct Black 171, Direct Black 19, Acid Black 1, Acid Black 191, MobayBlack SP, and Acid Black 2.

In other examples, the colorant can be a pigment. The pigment can beself-dispersed with a polymer, oligomer, or small molecule; or can bedispersed with a separate dispersant. Suitable pigments include, but arenot limited to, the following pigments available from BASF: Paliogen®)Orange, Heliogen® Blue L 6901F, Heliogen®) Blue NBD 7010, Heliogen® BlueK 7090, Heliogen® Blue L 7101F, Paliogen®) Blue L 6470, Heliogen®) GreenK 8683, and Heliogen® Green L 9140. The following black pigments areavailable from Cabot: Monarch® 1400, Monarch® 1300, Monarch®) 1100,Monarch® 1000, Monarch®) 900, Monarch® 880, Monarch® 800, and Monarch®)700. The following pigments are available from CIBA: Chromophtal®)Yellow 3G, Chromophtal®) Yellow GR, Chromophtal®) Yellow 8G, Igrazin®Yellow 5GT, Igralite® Rubine 4BL, Monastral® Magenta, Monastral®Scarlet, Monastral® Violet R, Monastral® Red B, and Monastral® VioletMaroon B. The following pigments are available from Degussa: Printex® U,Printex® V, Printex® 140U, Printex® 140V, Color Black FW 200, ColorBlack FW 2, Color Black FW 2V, Color Black FW 1, Color Black FW 18,Color Black S 160, Color Black S 170, Special Black 6, Special Black 5,Special Black 4A, and Special Black 4. The following pigment isavailable from DuPont: Tipure®) R-101. The following pigments areavailable from Heubach: Dalamar® Yellow YT-858-D and Heucophthal Blue GXBT-583D. The following pigments are available from Clariant: PermanentYellow GR, Permanent Yellow G, Permanent Yellow DHG, Permanent YellowNCG-71, Permanent Yellow GG, Hansa Yellow RA, Hansa Brilliant Yellow5GX-02, Hansa Yellow-X, Novoperm® Yellow HR, Novoperm® Yellow FGL, HansaBrilliant Yellow 10GX, Permanent Yellow G3R-01, Hostaperm® Yellow H4G,Hostaperm® Yellow H3G, Hostaperm® Orange GR, Hostaperm® Scarlet GO, andPermanent Rubine F6B. The following pigments are available from Mobay:Quindo® Magenta, Indofast® Brilliant Scarlet, Quindo® Red R6700, Quindo®Red R6713, and Indofast® Violet. The following pigments are availablefrom Sun Chemical: L74-1357 Yellow, L75-1331 Yellow, and L75-2577Yellow. The following pigments are available from Columbian: Raven®7000, Raven® 5750, Raven® 5250, Raven® 5000, and Raven® 3500. Thefollowing pigment is available from Sun Chemical: LHD9303 Black. Anyother pigment and/or dye can be used that is useful in modifying thecolor of the above described inks and/or ultimately, the printed part.

The colorant can be included in the conductive fusing ink and/or thesecond fusing ink to impart color to the printed object when the fusinginks are jetted onto the powder bed. Optionally, a set of differentlycolored fusing inks can be used to print multiple colors. For example, aset of fusing inks including any combination of cyan, magenta, yellow(and/or any other colors), colorless, white, and/or black fusing inkscan be used to print objects in full color. Alternatively oradditionally, a colorless fusing ink can be used in conjunction with aset of colored, non-fusing inks to impart color. In some examples, acolorless fusing ink can be used to coalesce the polymer powder and aseparate set of colored or black or white inks not containing a fusingagent can be used to impart color.

The components of the above described inks can be selected to give theinks good ink jetting performance and the ability to color the polymerpowder with good optical density. Besides the transition metals, fusingagents, colorants and other ingredients described above, the inks canalso include a liquid vehicle. In some examples, the liquid vehicleformulation can include water and one or more co-solvents present intotal at from 1 wt % to 50 wt %, depending on the jetting architecture.Further, one or more non-ionic, cationic, and/or anionic surfactant canoptionally be present, ranging from 0.01 wt % to 20 wt %. In oneexample, the surfactant can be present in an amount from 5 wt % to 20 wt%. The liquid vehicle can also include dispersants in an amount from 5wt % to 20 wt %. The balance of the formulation can be purified water,or other vehicle components such as biocides, viscosity modifiers,materials for pH adjustment, sequestering agents, preservatives, and thelike. In one example, the liquid vehicle can be predominantly water. Insome examples, a water-dispersible or water-soluble fusing agent can beused with an aqueous vehicle. Because the fusing agent is dispersible orsoluble in water, an organic co-solvent is not necessary to solubilizethe fusing agent. Therefore, in some examples the inks can besubstantially free of organic solvent. However, in other examples aco-solvent can be used to help disperse other dyes or pigments, orimprove the jetting properties of the ink. In still further examples, anon-aqueous vehicle can be used with an organic-soluble ororganic-dispersible fusing agent.

In certain examples, a high boiling point co-solvent can be included inthe inks. The high boiling point co-solvent can be an organic co-solventthat boils at a temperature higher than the temperature of the powderbed during printing. In some examples, the high boiling point co-solventcan have a boiling point above 250 ° C. In still further examples, thehigh boiling point co-solvent can be present in the ink at aconcentration from about 1 wt % to about 4 wt %.

Classes of co-solvents that can be used can include organic co-solventsincluding aliphatic alcohols, aromatic alcohols, dials, glycol ethers,polyglycol ethers, caprolactams, formamides, acetamides, and long chainalcohols. Examples of such compounds include primary aliphatic alcohols,secondary aliphatic alcohols, 1,2-alcohols, 1,3-alcohols, 1,5-alcohols,ethylene glycol alkyl ethers, propylene glycol alkyl ethers, higherhomologs (C₆-C₁₂) of polyethylene glycol alkyl ethers, N-alkylcaprolactams, unsubstituted caprolactams, both substituted andunsubstituted formamides, both substituted and unsubstituted acetamides,and the like. Specific examples of solvents that can be used include,but are not limited to, 2-pyrrolidinone, N-methylpyrrolidone,2-hydroxyethyl-2-pyrrolidone, 2-methyl-1,3-propanediol, tetraethyleneglycol, 1,6-hexanediol, 1,5-hexanediol and 1,5-pentanediol.

One or more surfactants can also be used, such as alkyl polyethyleneoxides, alkyl phenyl polyethylene oxides, polyethylene oxide blockcopolymers, acetylenic polyethylene oxides, polyethylene oxide(di)esters, polyethylene oxide amines, protonated polyethylene oxideamines, protonated polyethylene oxide amides, dimethicone copolyols,substituted amine oxides, and the like. The amount of surfactant addedto the formulation of this disclosure may range from 0.01 wt % to 20 wt%. Suitable surfactants can include, but are not limited to, liponicesters such as Tergitol™ 15-S-12, Tergitol™ 15-S-7 available from DowChemical Company, LEG-1 and LEG-7; Triton™ X-100; Triton™ X-405available from Dow Chemical Company ; and sodium dodecylsulfate.

Consistent with the formulation of this disclosure, various otheradditives can be employed to optimize the properties of the inkcompositions for specific applications. Examples of these additives arethose added to inhibit the growth of harmful microorganisms. Theseadditives may be biocides, fungicides, and other microbial agents, whichare routinely used in ink formulations. Examples of suitable microbialagents include, but are not limited to, NUOSEPT® (Nudex, Inc.),UCARCIDE™ (Union carbide Corp.), VANCIDE® (R.T. Vanderbilt Co.), PROXEL®(ICI America), and combinations thereof.

Sequestering agents, such as EDTA (ethylene diamine tetra acetic acid),may be included to eliminate the deleterious effects of heavy metalimpurities, and buffer solutions may be used to control the pH of theink. From 0.01 wt % to 2 wt %, for example, can be used. Viscositymodifiers and buffers may also be present, as well as other additives tomodify properties of the ink as desired. Such additives can be presentat from 0.01 wt % to 20 wt %.

In one example, the liquid vehicle can include the components andamounts as shown in Table 1:

TABLE 1 Ingredients Wt (%) 2-Pyrrolidinone 50-752-Methyl-1,3-Propanediol  5-12 Tetraethylene glycol  5-12 LEG-1  5-10Surfynol ® CT151 surfactant from Air Products and 0.2-1.2 Chemicals,Inc. Zonyl ® FSO fluorosurfactant from DuPont 0.01-1   SMA1440H 1-5 Trisbase 0.1-1  

In another example, the liquid vehicle can include the components andamounts as shown in Table 2:

TABLE 2 Ink Components Wt (%) 2-Pyrrolidinone 50-99.9 Crodafos N3 ™surfactant from Croda 0.1-5  

In yet another example, the liquid vehicle can include the componentsand amounts as shown in Table 3:

TABLE 3 Component Wt % 2-methyl-1,3-propanediol   10-40 Crodafos N3 ™surfactant from Croda 0.1-5 Tergitol ™ 15-S-12 surfactant from DowChemical Company 0.1-3 Zonyl ® FSO-100 fluorosurfactant from DuPont0.5-5 Proxel ™ GXL (20% as is) biocide from Lonza 0.1-1

In still another example, the liquid vehicle can include the componentsand amounts as shown in Table 4:

TABLE 4 Component Wt % 2-Hydroxyethyl-2-Pyrrolidone 5-20 Dantocol ™ DHEbonding agent from Lonza 30-80  LEG 1-20 Crodafos N3 ™ surfactant fromCroda 1-20 Surfynol ® SEF (75% as is) surfactant from Air Products and1-10 Chemicals, Inc. Kordek ™ MLX (10% as is) biocide from 0.1-5   DowChemical Company Proxel ™ GXL (20% as is) biocide from Lonza 0.1-5  

In a further example, the liquid vehicle can include the components andamounts as shown in Table 5:

TABLE 5 Ink vehicle components Wt % Tripropylene glycol 20-601-(2-Hydroxyethyl)-2-imidazolidinone 20-40 LEG-1 0.5-5   Crodafos N3 ™surfactant from Croda 1-6 Tergitol ™ 15-S-7 surfactant from Dow ChemicalCompany 1-6 Zonyl ® FSO fluorosurfactant from DuPont 0.1-1.2 Proxel ™GXL biocide from Lonza 0.1-1.2

In yet another example, the liquid vehicle can include the componentsand amounts shown in Table 6:

TABLE 6 Ink vehicle components Wt % Ethylene glycol 65-96.9 Ethanol3-20  Isopropyl alcohol 0.1-15 

It is noted the liquid vehicle formulations of Tables 1 to 6 areprovided by example only and other formulations with similar propertiescan likewise be formulated in accordance with the present technology

The present technology also extends to methods of making 3-dimensionalprinted parts having integrated strain sensors. The methods can use anyof the materials described above. FIG. 5 is a flowchart of an exemplarymethod 500 of making a 3-dimensional printed part having an integratedstrain sensor. The method includes dispensing a conductive fusing inkonto a first area of a layer of thermoplastic polymer particles, whereinthe conductive fusing ink includes a transition metal 510; dispensing asecond fusing ink onto a second area of the layer of thermoplasticpolymer particles, wherein the second fusing ink includes a fusing agentcapable of absorbing electromagnetic radiation to produce heat 520; andfusing the first and second areas with electromagnetic radiation to forma resistor in the first area and a part body in the second area, whereinthe resistor includes a matrix of sintered transition metal particlesinterlocked with a matrix of fused thermoplastic polymer particles, andthe part body includes fused thermoplastic polymer particles 530.

In further examples, methods of making 3-dimensional printed partshaving integrated strain sensors can include dispensing additional inksonto the thermoplastic polymer particles. For example, a pretreat inkcan be dispensed onto the polymer particles before dispensing theconductive fusing ink. The pretreat ink can include a halogen salt suchas sodium chloride or potassium chloride to remove dispersing agentsfrom the transition metal particles in the conductive fusing ink.Colored inks can also be dispensed onto the polymer particles to providevisible colors to the printed part.

In some examples, the fusing inks and other inks can be dispensed by inkjetting. This can be performed by a thermal ink jet printing system or apiezo ink jet printing system. Any other suitable method of dispensingthe inks onto the polymer particles can also be used.

In additional examples, the methods described herein can be performedusing a powder bed 3-dimensional printing system. In one example, thebed of the thermoplastic polymer particles can be formed by introducingpolymer powder from a polymer powder supply and rolling the powder in athin layer using a roller. The conductive fusing ink and second fusingink can be jetted using ink jet print heads. The amount of conductiveink printed can be calibrated based on the concentration of transitionmetal in the ink, the temperature boosting capacity of the transitionmetal, the desired conductivity of the resulting conductive compositematerial to be printed, among other factors. Similarly, the amount ofthe second fusing ink printed can be calibrated based the concentrationof fusing agent, temperature boosting capacity of the fusing agent, andother factors. In some examples, the amount of fusing ink printed can besufficient to contact a fusing agent with the entire layer of polymerpowder. For example, if each layer of polymer powder is 100 micronsthick, then the fusing ink can penetrate at least 100 microns into thepolymer powder. Thus the fusing agents can heat the polymer powderthroughout the entire layer so that the layer can coalesce and bond tothe layer below. After forming a solid layer, a new layer of loosepowder can be formed, either by lowering the powder bed or by raisingthe height of the roller and rolling a new layer of powder.

The entire powder bed can be preheated to a temperature below themelting or softening point of the polymer powder. In one example, thepreheat temperature can be from about 10° C. to about 30° C. below themelting or softening point. In another example, the preheat temperaturecan be within 50° C. of the melting of softening point. In a particularexample, the preheat temperature can be from about 160° C. to about 170°C. and the polymer powder can be nylon 12 powder. In another example,the preheat temperature can be about 90° C. to about 100° C. and thepolymer powder can be thermoplastic polyurethane. Preheating can beaccomplished with one or more lamps, an oven, a heated support bed, orother types of heaters. In some examples, the entire powder bed can beheated to a substantially uniform temperature.

The powder bed can be irradiated with a fusing lamp. Suitable fusinglamps for use in the methods described herein can include commerciallyavailable infrared lamps and halogen lamps. The fusing lamp can be astationary lamp or a moving lamp. For example, the lamp can be mountedon a track to move horizontally across the powder bed. Such a fusinglamp can make multiple passes over the bed depending on the amount ofexposure needed to coalesce each printed layer. The fusing lamp can beconfigured to irradiate the entire powder bed with a substantiallyuniform amount of energy. This can selectively coalesce the printedportions with fusing inks leaving the unprinted portions of the polymerpowder below the melting or softening point.

In one example, the fusing lamp can be matched with the fusing agents inthe fusing inks so that the fusing lamp emits wavelengths of light thatmatch the peak absorption wavelengths of the fusing agents. A fusingagent with a narrow peak at a particular near-infrared wavelength can beused with a fusing lamp that emits a narrow range of wavelengths atapproximately the peak wavelength of the fusing agent. Similarly, afusing agent that absorbs a broad range of near-infrared wavelengths canbe used with a fusing lamp that emits a broad range of wavelengths.Matching the fusing agent and the fusing lamp in this way can increasethe efficiency of coalescing the polymer particles with the fusing agentprinted thereon, while the unprinted polymer particles do not absorb asmuch light and remain at a lower temperature.

Depending on the amount of fusing agent present in the polymer powder,the absorbance of the fusing agent, the preheat temperature, and themelting or softening point of the polymer, an appropriate amount ofirradiation can be supplied from the fusing lamp. In some examples, thefusing lamp can irradiate each layer from about 0.5 to about 10 secondsper pass.

In further examples, methods of making 3-dimensional printed partshaving integrated strain sensors can include tuning the resistance ofthe 3-dimensional printed resistor to a desired range. As explainedabove, the resistor can have the form of a conductive composite with amatrix of fused thermoplastic polymer particles interlocked with amatrix of sintered transition metal particles. The resistance of theconductive composite can be tuned in a variety of ways. For example, theresistance can be affected by the type of transition metal in theconductive fusing ink, the concentration of the transition metal in theconductive fusing ink, the amount of conductive fusing ink dispensedonto the powder bed, the cross section and length of the resistor, andso on. When the conductive fusing ink is dispensed by ink jetting, theamount of conductive fusing ink dispensed can be adjusted by changingprint speed, drop weight, number of slots from which the ink is fired inthe ink jet printer, and number of passes printed per powder layer. Incertain examples, a conductive composite element can have a resistancefrom 1 ohm to 5 Mega ohms.

Sufficient conductivity can be achieved by dispensing a sufficientamount of the transition metal onto the powder bed. In some examples, asufficient mass of the transition metal per volume of the conductivecomposite can be used to achieve conductivity. For example, the mass oftransition metal per volume of conductive composite can be greater than1 mg/cm³, greater than 10 mg/cm³, greater than 50 mg/cm³, or greaterthan 100 mg/cm³. In a particular example, the mass of transition metalper volume of the conductive composite can be greater than 140 mg/cm³.In further examples, the mass of transition metal per volume ofconductive composite can be from 1 mg/cm³ to 1000 mg/cm³, from 10 mg/cm³to 1000 mg/cm³, from 50 mg/cm³ to 500 mg/cm³, or from 100 mg/cm³ to 500mg/cm³.

In certain examples, a smaller amount of transition metal can bedispensed to achieve surface conductivity, and a larger amount oftransition metal can be applied to achieve bulk conductivity in theconductive composite. Thus, in some examples a smaller amount ofconductive fusing ink can be printed on a single layer of polymerparticles to form a resistor that has conductivity across the surface ofthe layer, i.e., in the x-y plane. In some examples, resistors withconductivity in the x-y plane can be formed with a mass of transitionmetal per volume of conductive composite that is greater than 1 mg/cm³or greater than 10 mg/cm³. In further examples, such resistors can beformed with a mass of transition metal per volume of conductivecomposite from 1 mg/cm³ to 1000 mg/cm³, 10 mg/cm³ to 500 mg/cm³, or 30mg/cm³ to 200 mg/cm³. However, such resistors may not have sufficientconductivity in the z-axis direction, or in other words, through thebulk of the layer. As used herein, the “z-axis” refers to the axisorthogonal to the x-y plane. For example, in 3-dimensional printingmethods that use a powder bed that lowers after each layer is printed,the powder bed is lowered in the z-axis direction.

In some examples, a resistor that is conductive only in the x-y planecan be sufficient. This is the case when the resistor is formed parallelto the layers of the 3-dimensional printed part. However, methodsaccording to the present technology can also be used to print resistorsthat are conductive in the z-axis direction. By dispensing a largeramount of conductive fusing ink onto the layer of polymer particles, theconductive ink can penetrate through the layer and conductivity betweenlayers in the z-axis direction can be achieved. In some examples,resistors that are conductive in the z-axis direction can be formed witha mass of transition metal per volume of conductive composite that isgreater than 50 mg/cm³ or greater than 100 mg/cm³. In further examples,such resistors can be formed with a mass of transition metal per volumeof conductive composite from 50 mg/cm³ to 1000 mg/cm³, 100 mg/cm³ to1000 mg/cm³, or 140 mg/cm³ to 500 mg/cm³.

In some examples, the amount of transition metal dispensed onto thepowder bed can be adjusted by printing the conductive fusing ink inmultiple passes. In one example, a single pass of an inkjet print headcan be sufficient to dispense enough transition metal to achieve surfaceconductivity. However, in some cases, a single pass is not sufficient toachieve conductivity in the z-axis direction. Additional passes can beapplied to increase the amount of transition metal in the transitionmetal composite. A sufficient number of passes can be used to achieveconductivity in the z-axis direction. In one example, three or morepasses can be used to form a conductive composite with conductivity inthe z-axis direction. In further examples, the amount of transitionmetal dispensed can be adjusted by adjusting the drop weight of theinkjet printhead either through resistor design or by changing firingparameters. Thus, with a greater drop weight, a greater amount of theconductive fusing ink can be printed with each drop fired. However, insome cases jetting too large an amount of ink in a single pass can leadto lower print quality because of ink spreading. Therefore, in someexamples multiple passes can be used to print more of the conductivefusing ink with better print quality

In a particular example, a 3-dimensional printed part can be formed asfollows. An inkjet printer can be used to print a first pass includingprinting a conductive fusing ink onto a first portion of the powder bedand printing a second fusing ink onto a second portion of the powderbed. A curing pass can then be performed by passing a fusing lamp overthe powder bed to fuse the polymer particles and sinter transition metalparticles in the conductive curing ink. Then, one or more additionalpasses can be performed of printing the conductive fusing ink onto thefirst portion of the powder bed to increase the amount of transitionmetal. Each pass of printing the conductive fusing ink can be followedby a curing pass with the fusing lamp. The number of passes used candepend on the desired conductivity, the contone level of the printingpasses (referring to the density of ink per area deposited on eachpass), the type of transition metal in the conductive fusing ink,concentration of transition metal in the conductive fusing ink,thickness of the layer of polymer powder being printed, and so on.

Accordingly, the methods of the present technology can be used to make3-dimensional printed parts with integrated strain sensors that areoriented in any direction. As explained above, a resistor can be formedin the x-y plane with respect to the layers of the 3-dimensional printedpart using a relatively smaller amount of conductive fusing ink, whileresistors oriented in the z-axis direction can be formed by using arelatively greater amount of conductive fusing ink on each layer. In oneexample, the resistor can be oriented at least partially in the z-axisdirection with respect to the layers of the 3-dimensional printed part.As used herein, “at least partially in the z-axis direction” refers toany direction that has at least a non-zero component on the z-axis.Therefore, resistors can be formed parallel to the z-axis or diagonal tothe z-axis using the methods described herein.

The 3-dimensional printing methods described herein can be used tomanufacture a wide variety of complex part shapes. The resistors printedas integrated strain sensors can similarly have a wide variety ofshapes. In some examples, the resistor can have a serpentine shape withmultiple turns along the length of the resistor. In some cases,increasing the length of the resistor can increase the overallresistance, while increasing the number of turns in the resistor canimprove the sensitivity of the strain sensor. Other resistor shapes canalso be used.

3-dimensional printed parts can also be made with multiple integratedresistors. For example, multiple resistors can be included to measurestrain on multiple portions of the part. In another example, tworesistors can be placed on opposite sides of a part under flexing stressso that one resistor can be compressed and the other resistor can beextended by the stress. Measurements from both resistors can be used toincrease sensitivity and accuracy of the strain sensor.

FIGS. 6A-6B show an example of a 3-dimensional printed part 600 thatincorporates some of the features mentioned above. Particularly, the3-dimensional printed part includes multiple resistors 610, 620 thathave a curved shape and that are printed so that portions of theresistors are oriented in the z-axis direction with respect to thelayers making up the printed part. The part includes a first resistor610 embedded in the part near the inner curved surface 630 of the part.A second resistor 620 is embedded in the part near the outer curvedsurface 640 of the part. The first and second resistors are connected toelectrical contacts 650 that are exposed at the surfaces of the part.The electrical contacts can be used to connect the resistors to ameasurement circuit to measure changes in resistance of the resistor.

It is noted that, as used in this specification and the appended claims,the singular forms “a,” “an,” and “the” include plural referents unlessthe context clearly dictates otherwise.

As used herein, “liquid vehicle” or “ink vehicle” refers to a liquidfluid in which colorant is placed to form an ink. A wide variety of inkvehicles may be used with the systems and methods of the presentdisclosure. Such ink vehicles may include a mixture of a variety ofdifferent agents, including, surfactants, solvents, co-solvents,anti-kogation agents, buffers, biocides, sequestering agents, viscositymodifiers, surface- active agents, water, etc. Though not part of theliquid vehicle per se, in addition to the colorants and fusing agents,the liquid vehicle can carry solid additives such as polymers, latexes,UV curable materials, plasticizers, salts, etc.

As used herein, “colorant” can include dyes and/or pigments.

As used herein, “dye” refers to compounds or molecules that absorbelectromagnetic radiation or certain wavelengths thereof. Dyes canimpart a visible color to an ink if the dyes absorb wavelengths in thevisible spectrum.

As used herein, “pigment” generally includes pigment colorants, magneticparticles, aluminas, silicas, and/or other ceramics, organo-metallics orother opaque particles, whether or not such particulates impart color.Thus, though the present description primarily exemplifies the use ofpigment colorants, the term “pigment” can be used more generally todescribe not only pigment colorants, but other pigments such asorganometallics, ferrites, ceramics, etc. In one specific aspect,however, the pigment is a pigment colorant.

As used herein, “soluble,” refers to a solubility percentage of morethan 5 wt %.

As used herein, “ink jetting” or “jetting” refers to compositions thatare ejected from jetting architecture, such as ink-jet architecture.Ink-jet architecture can include thermal or piezo architecture.Additionally, such architecture can be configured to print varying dropsizes such as less than 10 picoliters, less than 20 picoliters, lessthan 30 picoliters, less than 40 picoliters, less than 50 picoliters,etc.

As used herein, the term “substantial” or “substantially” when used inreference to a quantity or amount of a material, or a specificcharacteristic thereof, refers to an amount that is sufficient toprovide an effect that the material or characteristic was intended toprovide. The exact degree of deviation allowable may in some casesdepend on the specific context.

As used herein, the term “about” is used to provide flexibility to anumerical range endpoint by providing that a given value may be “alittle above” or “a little below” the endpoint. The degree offlexibility of this term can be dictated by the particular variable anddetermined based on the associated description herein.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Concentrations, amounts, and other numerical data may be expressed orpresented herein in a range format. It is to be understood that such arange format is used merely for convenience and brevity and thus shouldbe interpreted flexibly to include not only the numerical valuesexplicitly recited as the limits of the range, but also to includeindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. As anillustration, a numerical range of “about 1 wt % to about 5 wt %” shouldbe interpreted to include not only the explicitly recited values ofabout 1 wt % to about 5 wt %, but also include individual values andsub-ranges within the indicated range. Thus, included in this numericalrange are individual values such as 2, 3.5, and 4 and sub-ranges such asfrom 1-3, from 2-4, and from 3-5, etc. This same principle applies toranges reciting only one numerical value. Furthermore, such aninterpretation should apply regardless of the breadth of the range orthe characteristics being described.

EXAMPLE

The following illustrates an example of the present disclosure.

However, it is to be understood that the following is only illustrativeof the application of the principles of the present disclosure. Numerousmodifications and alternative compositions, methods, and systems may bedevised without departing from the spirit and scope of the presentdisclosure. The appended claims are intended to cover such modificationsand arrangements.

Example 1

A 3-dimensional printing system was used to print a strain sensor with aserpentine shaped resistor embedded in a rectangular polymer strip. Aconductive fusing ink, pretreat ink, and second fusing ink were printedfrom three separate ink jet pens. The conductive fusing ink was a silverink (Mitsubishi NBSIJ-MU01) containing silver nanoparticles. The silvernanoparticles had an average particle size of approximately 20 nm. Thepretreat ink was a solution of 3 wt % sodium chloride in water. Thesecond fusing ink included carbon black as the fusing agent and anaqueous ink vehicle.

The inks were jetted onto a bed of nylon (PA12) particles (Vestosint®x1556). The nylon particles had an average particle size ofapproximately 50 μm. The layer thickness was approximately 100 μm. Eachlayer was printed with the pretreat ink followed by the silver ink inthe portions that make up the resistor, and the carbon black fusing inkin the insulating portions. The inks were printed at contone levels of255 for the silver ink, 255 for the pretreat ink, and 15 for the carbonblack ink. 3 passes of the inks were performed for each layer. Aftereach pass with the inks, a curing pass was performed. In this example,the amount of solid silver dispensed onto the powder was 141 mg/cm³ ofthe powder layer; the amount of chloride salt dispensed was 23 mg/cm³ ofthe powder layer; and the amount of carbon black dispensed was 2.3mg/cm³ of the powder layer.

The printer powder supply and powder bed were filled with the nylonparticles. The supply temperature was set at 110 ° C. and the print bedtemperature was set at 130 ° C. A heater under the print bed was set at150 ° C. The print speed was set at 10 inches per second (ips) and thecure speed was set at 7 ips. Curing was performed using two 300 W bulbsplaced approximately 1 cm away from the surface of the powder bed.

Four copies of the strain sensor were printed simultaneously on a singlepowder bed. The four strain sensors were tested for sensitivity tochanging strains. Test leads were clipped to the electrical contact padson the sensors, and their resistances were measured with a benchtopdigital multi meter. The sensors were then bent by hand 90 degrees inthe direction that placed the resistors in tension, and theirresistances were measured again. The sensors were then bent back totheir original shape and the resistances were measured again. Themeasured resistances are shown in Table 7 below.

TABLE 7 R initial R in tension Sensor # (ohms) (ohms) R final (ohms) %delta R 1 4650 5375 4601 15.6% 2 3750 4700 3820 25.3% 3 1703 2098 174623.1% 4 3438 4780 3435   39%

Each resistor displayed a significant change in resistance when put intension. Further, each resistor returned to nearly its originalresistance when returned to its original shape. These properties makethe resistors useful as strain sensors that can measure strainsrepeatedly with sensitivity. It should be noted that the variation inresistance between sensors 1-4 is likely caused by variations intemperature in different areas of the powder bed during printing.

What is claimed is:
 1. A strain sensor, comprising: a resistor formed ofa matrix of sintered elemental transition metal particles interlockedwith a matrix of fused thermoplastic polymer particles; a firstelectrical contact at a first end of the resistor; and a secondelectrical contact at a second end of the resistor.
 2. The strain sensorof claim 1, wherein the elemental transition metal particles comprisesilver particles, copper particles, gold particles, or combinationsthereof.
 3. The strain sensor of claim 1, wherein the matrix of fusedthermoplastic polymer particles comprises a fusing agent selected fromcarbon black, a near-infrared absorbing dye, a near-infrared absorbingpigment, a tungsten bronze, a molybdenum bronze, metal nanoparticles, aconjugated polymer, or combinations thereof.
 4. The strain sensor ofclaim 1, wherein the resistor further comprises a halogen salt in thematrix of sintered elemental transition metal particles, the matrix offused thermoplastic polymer particles, or both.
 5. The strain sensor ofclaim 1, wherein the resistor has a resistance from 1 ohm to 1 Mega ohm.6. A 3-dimensional printed part having an integrated strain sensor,comprising: a part body formed of fused thermoplastic polymer particles;and a resistor formed of a matrix of sintered elemental transition metalparticles interlocked with a matrix of fused thermoplastic polymerparticles, wherein the matrix of fused thermoplastic polymer particlesis continuously fused to the fused thermoplastic polymer particles ofthe part body.
 7. The 3-dimensional printed part of claim 6, wherein theelemental transition metal particles comprise silver particles, copperparticles, gold particles, or combinations thereof.
 8. The 3-dimensionalprinted part of claim 6, wherein the fused thermoplastic polymerparticles comprise a fusing agent selected from carbon black, anear-infrared absorbing dye, a near-infrared absorbing pigment, atungsten bronze, a molybdenum bronze, metal nanoparticles, a conjugatedpolymer, or combinations thereof.
 9. The 3-dimensional printed part ofclaim 6, wherein the resistor further comprises a halogen salt in thematrix of sintered elemental transition metal particles, the matrix offused thermoplastic polymer particles, or both.
 10. The 3-dimensionalprinted part of claim 6, wherein the resistor has a resistance from 1ohm to 1 Mega ohm.
 11. The 3-dimensional printed part of claim 6,wherein the resistor is embedded in the part body.
 12. The 3-dimensionalprinted part of claim 6, wherein the part is formed of multiple layersof fused thermoplastic polymer particles stacked in a z-axis direction,and wherein the resistor is oriented at least partially in the z-axisdirection.
 13. A method of making a 3-dimensional printed part having anintegrated strain sensor, the method comprising: dispensing a conductivefusing ink onto a first area of a layer of thermoplastic polymerparticles, wherein the conductive fusing ink comprises a transitionmetal; dispensing a second fusing ink onto a second area of the layer ofthermoplastic polymer particles, wherein the second fusing ink comprisesa fusing agent capable of absorbing electromagnetic radiation to produceheat; and fusing the first and second areas with electromagneticradiation to form a resistor in the first area and a part body in thesecond area, wherein the resistor comprises a matrix of sinteredtransition metal particles interlocked with a matrix of fusedthermoplastic polymer particles, and the part body comprises fusedthermoplastic polymer particles.
 14. The method of claim 13, wherein theresistor is formed at least partially oriented in a z-axis directionsuch that the resistor extends across multiple layers of the3-dimensional printed part.
 15. The method of claim 13, wherein thetransition metal is in the form of elemental transition metal particles.